The <operation> element enables you to perform additional transformations on the original data during rendering. The <operation> element has the following definition:

<!ELEMENT operation (parameter+) >
<!ATTLIST parameter
  name CDATA #REQUIRED
>

Currently this element is used in GeoRaster themes (described in GeoRaster Themes). You can perform some image processing operations on the original image, such as normalization, equalization, linear stretch, piecewise linear stretch, brightness and contrast adjustment, and threshold change.

Example 3-21 specifies the normalization operation with a GeoRaster theme.

<theme name="geor_theme">
    <jdbc_georaster_query jdbc_srid="0"
                          datasource="mvdemo"
                          georaster_table="dem"
                          georaster_column="georaster"
                          asis="false">
        select georaster from dem
    </jdbc_georaster_query>
    <operations>
        <operation name="normalize" />
    </operations>
</theme>

The following code segment shows a manual linear stretch operation. (For automatic linear stretch, include the <operation> element but no <parameter> elements.)

<operation name="linearstretch">
    <parameter name="autostretch"
               value="false" />
    <parameter name="lowstretch"
               value="50" />
    <parameter name="highstretch"
               value="150" />
</operation>

The following table lists the image processing operations, their <operation> element name keyword values, and (where relevant) associated <parameter> element values.